A power supply produces, from an input DC power of a certain voltage, an output DC power of a desired voltage. The produced output DC power is supplied as source power to various electronic circuit devices, such as a system LSI and a memory device.
Such a power supply is also called a DC-DC converter for converting an input DC voltage to an output DC voltage that differs from the input DC voltage. A recent demand for saving electric power has been imposed on the power supply as well. Thus, it has been required to produce an output power with optimum efficiency depending on the load state of an electronic circuit device to which the output power is supplied. To meet the demand for saving electric power, in particular, a recent LSI is designed to have a plurality of power domains and to finely control a power supplied state and a power stopped state per power domain. Therefore, the load state of the power supply changes in a complex way. Correspondingly, it is required that the power supply is flexibly adaptable for changes of the load state and may efficiently produce power. For example, the power supply is required to detect a light load and to produce a power adapted for the detected load with high efficiency. Japanese Unexamined Patent Application Publication No. 2000-245150 discloses a switching power supply circuit intended to meet that requirement.
In the power supply, for example, a high-side output transistor and a low-side output transistor are alternately switched over such that electric charges are supplied to an output terminal coupled to a junction between both the transistors. Further, an output voltage is controlled to be held at a target value by prolonging a driving time of the high-side output transistor when an output load increases, and by shortening the driving time thereof when the output load reduces.
It is desired that the power supply detects a reduction of the output load and makes a shift to a low consumption current mode upon the detection. To that end, the power supply monitors a current of the high-side output transistor and detects a light load when the current decreases.
However, the method of detecting a current reduction in the output transistor has a difficulty in detecting the current reduction with high accuracy, and detection of a low load may not be performed with high accuracy. Further, the method of monitoring the current is poor in response.